The present invention relates to a method for producing a Group III-V compound semiconductor used for light-emitting devices, electronic devices and the like and also relates to a semiconductor laser device for emitting laser light at a short wavelength by using the same.
In recent years, semiconductor light-emitting devices (semiconductor laser devices, in particular) for emitting light in a short-wavelength region ranging from the spectrum of ultraviolet to the spectrum of blue have been vigorously researched and developed. This is because such devices ensure an increase in recording density of an optical disk or resolution of a laser printer and are applicable to various types of optical measuring instruments, medical devices, display devices and illuminators.
Examples of materials that can emit light at such a short-wavelength region include Group III-V compound semiconductors containing nitrogen. According to Applied Physics Letters, Vol. 70 (1997) pp. 1417-1419, a semiconductor laser device, including an Si-doped InGaN multi-quantum well active layer, can continuously oscillate at a wavelength of around 406 nm and at room temperature. As described in this document, the operating life thereof is 27 hours on the conditions that the temperature is 20xc2x0 C. and the output power is 1.5 mW.
However, the operating life of the conventional semiconductor laser device using the InGaN-based compound semiconductors is still far from satisfactorily long, because in practice, an operating life exceeding 10,000 hours is often required.
In view of these problems, the present invention was made to procure an expected operating life for a semiconductor laser device emitting light at a short wavelength by using Group III-V compound semiconductors containing indium and nitrogen.
To accomplish this object, according to the present invention, in producing a Group III-V compound semiconductor containing indium and nitrogen by a metalorganic vapor phase epitaxy (MOVPE) technique, a rare gas is used as a carrier gas or nitrogen atoms or molecules contained in a nitrogen source are excited. On the other hand, in producing such a Group III-V compound semiconductor by a molecular beam epitaxy (MBE) technique, the nitrogen atoms, contained in the nitrogen source, are excited.
Specifically, a first method according to the present invention is a method for producing a semiconductor by growing a compound semiconductor on a substrate held by a susceptor provided in a reaction chamber in accordance with an MOVPE technique. The method includes the steps of: supplying a Group III source gas containing indium and a Group V source gas containing nitrogen into the reaction chamber; and mixing the Group III and Group V source gases, supplied into the reaction chamber, with each other, and supplying a rare gas as a carrier gas into the reaction chamber so as to carry the mixed source gas onto the upper surface of the substrate.
In accordance with the first method, a rare gas is used as a carrier gas for carrying a mixed source gas onto the upper surface of a substrate. Accordingly, as compared with using nitrogen gas as a carrier gas, the Group V source gas such as ammonium gas can be dissolved with higher efficiency. Also, since a rare gas has lower thermal conductivity than that of nitrogen gas, the generation of vacancies can be suppressed in nitrogen atoms in a crystal. As a result, the density of n-type residual carriers, which is ordinarily increased by the existence of nitrogen vacancies, can be reduced, and the resistance can be increased. Thus, even if an active layer is formed for a light-emitting device or the like by using a Group III-V compound semiconductor containing indium and nitrogen having respective carrier densities reduced, the active layer is not damaged easily. Consequently, an expected operating life can be secured.
In one embodiment of the present invention, the rare gas is preferably argon. In such an embodiment, the residual carrier density of the Group III-V compound semiconductor containing indium and nitrogen can be reduced with more certainty, because argon gas is available relatively easily compared with other rare gases.
A second method according to the present invention is a method for producing a semiconductor by growing a compound semiconductor on a substrate in accordance with an MOVPE technique. The method includes the steps of: supplying a Group III source gas containing indium onto the substrate; and supplying a Group V source gas containing nitrogen onto the substrate. The step of supplying the Group V source gas includes the step of making nitrogen atoms or molecules, contained in the Group V source gas, excited.
In accordance with the second method, in the step of supplying a Group V source gas containing nitrogen into a reaction chamber, nitrogen atoms or molecules, contained in the Group V source gas, are excited. Accordingly, the nitrogen atoms, having a high vapor pressure, are introduced more easily into the faces of crystals grown on the substrate. As a result, the generation of nitrogen vacancies can be suppressed in a crystal, and the density of n-type residual carriers, which is ordinarily increased by nitrogen vacancies, can be reduced. Thus, if an active layer is formed for a light-emitting device or the like by using a Group III-V compound semiconductor containing indium and nitrogen having respective carrier densities reduced, the deterioration of operation characteristics, caused by residual carriers, can be suppressed. Consequently, the active layer is not damaged easily, and an expected operating life can be secured.
In one embodiment of the present invention, the Group V source gas is preferably nitrogen gas. In such an embodiment, hydrogen is less likely to be absorbed into crystals as compared with using ammonium as a nitrogen source. As a result, the quality of crystals can be further improved.
A third method according to the present invention is a method for producing a semiconductor by growing a compound semiconductor, containing at least indium and nitrogen, on a substrate in accordance with an MBE technique. The method includes the steps of: irradiating an indium molecular beam onto the upper surface of the substrate; and irradiating a molecular beam containing nitrogen onto the upper surface of the substrate. The step of irradiating the molecular beam containing nitrogen further includes the step of making nitrogen atoms, contained in the molecular beam containing nitrogen, excited.
In accordance with the third method, nitrogen atoms contained in a Group V source gas are excited in the step of irradiating a molecular beam of the Group V source gas containing nitrogen onto the upper surface of a substrate. Accordingly, the nitrogen atoms having a high vapor pressure are more likely to be introduced into the faces of crystals grown on the substrate. Thus, the same effects as those attained by the second method of the present invention can also be attained.
In the second or third method of the present invention, a radio frequency plasma generation technique or an electron cyclotron resonance plasma generation technique is preferably used in the step of exciting. In such an embodiment, since the nitrogen atoms or molecules are turned into plasma, active nitrogen can be produced with more certainty.
A semiconductor laser device according to the present invention includes: a first cladding layer of a first conductivity type formed on a substrate; an active layer formed on the first cladding layer and made of a compound semiconductor containing at least indium and nitrogen; and a second cladding layer of a second conductivity type formed on the active layer. The density of residual carriers in the active layer is less than 1xc3x971017 cmxe2x88x923.
In the semiconductor laser device of the present invention, the density of residual carriers in the active layer, made of a compound semiconductor containing at least indium and nitrogen, is less than 1xc3x971017 cmxe2x88x923. Accordingly, the deterioration of operation characteristics, caused by residual carriers, can be suppressed and the active layer is not damaged easily. As a result, an expected operating life can be secured and the reliability of the device can be improved.